Forbidden Gamma --> e+e-: Show Conservation Violation

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Homework Statement



Show that although gamma rays can have energies greater than the energy of a e+e- pair at rest, the process gamma --> e+e- is forbidden.

Homework Equations



Noether's theorem

The Attempt at a Solution



I know that to show that the process is forbidden, I should be able to show that it violates a conservation principle. But I'm not at all sure what formalism to use for the problem. Can someone give me a nudge in the right direction?
 

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Energy may conserved in this process but what other major conserved quantities do we have? HINT: The one that isn't conserved here, you learned about in introductory physics. Remember, the resulting particles are at rest.
 
Ok, so it turns out that I was misreading the problem. All that I need to do (if I am now reading correctly, lol) is to prove that pair production can't occur in empty space! That's not bad at all; the requirements of energy conservation demand that another nucleus be present to absorb some of the photon's linear momentum (without another nucleus, conservation of linear momentum is violated). It's a quick proof. Thanks for the help. Sometimes I just need to actually read my problems more carefully...